I guess that before you did the domain mapping you redirected the site on Godaddy to redirect to nbdnation.wordpress.com. When you set up the domain mapping part of what it does is redirect http://nbdnation.wordpress.com URLs to http://nbdnation.com URLS, so that existing links work. If this domain is sending you back to the ,wordpress.com URL you will get the infinite loop until the domain updates propagate across the web.

After I set nbdnation.com as my primary domain, I receive a redirect loop error message on my browser. ( i.e. "[th]is webpage has a redirect loop
The webpage at http://nbdnation.wordpress.com/ has resulted in too many redirects. Clearing your cookies for this site or allowing third-party cookies may fix the problem. If not, it is possibly a server configuration issue and not a problem with your computer.") Also, the address bar in this browser window displays nbdnation.wordpress.com, not nbdnation.com

I did a search in this forum, and it appears I've followed the directions properly.

I called GoDaddy, and they said the name servers were properly updated.

It's been about 24 hours, and I understand it can take up to 48 hours. Nonetheless, I figured I'd solicit help, in case I made a mistake.